Functional Stratification of Microbes in the Baltic Sea
The Baltic Sea off the coasts of Sweden and Denmark is highly variable with regard to environmental conditions throughout the water column. Surface waters tend to have relatively low salinity but high dissolved oxygen with the drainage basin inputting large amounts of freshwater. The catchment is also the source of a large amount of anthropogenic pollution, resulting in eutrophication and spring and summer phytoplankton blooms. The blooms reduce dissolved oxygen levels considerably and create anoxic waters that are dense and settle below the halocline (1). The sea is also intermittently infused with highly oxygenated salt water from the North Sea, resulting in drastic stratification. The result of these contrasting processes are a benthic zone that is hypoxic with anoxic sediments residing below, a relatively oxygen, salt, and nutrient rich metalimnion, and a high oxygen, low salt epilimion. The chemocline is one of the steepest in the world with an exaggerated gradient for oxygen and various nutrients like phosphate and nitrate. Due to the scale of the gradients and the depth of the Baltic sea the gradients are also some the most persistent with little to no vertical mixing occurring between the benthic zone and the metalimnion (1). Due to the varying nature of water composition at differing depths, the possible niches for microbes are vast and the Thureborn group predicted functional stratification of microbes across the condition gradients. The study took place at one of the deepest locations in the Baltic Sea: Landsort Deep. Landsort Deep exhibits a relative consistent chemocline, as it is resistant to change from seasonal mixing because of its depth. Sampling and Analysis Metagenomes were sampled from from the four distinct layers of the Baltic Sea: the sediment of the benthic zone, the water of the benthic zone, the water of the metalimnion, and the water of the epilimnion. 454 GS FLX Titanium sequencing was run on each of the samples coming from a different lake layer, producing 417 Mb of sequencing data comprising 1,205,630 reads (1). Quality control techniques were then utilized, including the deletion of duplicate reads using CDHIT-454, to refine the data. In each dataset approximately 40-50% of the reads could be assigned to taxa while 20% could be assigned a putative function based upon reference to the SEED database (1). The group also analyzed for environmental influence as the highly variable conditions create very distinct niches. They also wanted to isolate environmental variables and determine the influence each was imparting on the community at that level. The group followed a similar protocol and performed a correspondence analysis on the reads that were assigned to bacterial or archaeal families. Not surpisingly, the CA1 (first ordinal axis) representing the environmental parameters with the strongest effect corresponded best with dissolved oxygen, salinity, and termperature. These are large drivers of what makes an environment habitable to a species and so this results might be expected. Stratification was most evident with respect to predicted gene function with 76% of total variation corresponding to CA1 (1). There was also substantial correspondence for community taxa at around 59%. Nitrogen and Sulfur Metabolism After witnessing distinct stratification of the microbes at Landsort Deep the Thurborn group wanted to investigate how this separation was represented in the N-dependent and S-dependent functional capactieis. This is specifically important in Landsort Deep due to the excessive loading of nutrients that result in eutrophication which encourage phytoplankton blooms. The group discovered that there were differences between communities and their ability to perform denitrification and or nitrite/nitrate ammonification. The largest amount of denitrfication capacity was found within the 400 m depth community. Therefore it was not surprising to find almost 1% of the reads at 400 m matched to Epsilonproteobacterium ''Sulfurimonas gotlandica, ''which has the ablity chemolithotrophic dentrification coupled with sulphur oxidation (1). Sulfur-oxidising Epsilonprotobacteria are an important catalyst of chemoautotrophic dentrification and CO2 fixation, and this has been shown in other high nutrient deep water environments. The Thurborn group goes on to analysis the microbial communities at many levels within the water column and their prospective reason for residing in that location. To learn more about each microbial subset and the environments they inhabit follow the reference at the bottom of this page. Environment is Primary Influence As the Thureborn group collected and analyzed data it became clear that functional capacity as well as community taxa were influencing. Landsort Deep is unique in the sense that the lower depths do not mix vertically, meaning those layers of water are relatively isolated from the rest of the water body. The group was extremely interested to see if this isolation led a microbial community below the halocline that had been evolving in isolation and therefore formed a unique community. The group did indeed find statistically significant separations of taxa at the different depths (1). Using phylogenetics trees to map out the relationships of the metabolic communities the Thuregood group was able to show metabolic stratification. The trees group the 400 m Landsort deep microbial communities with others that reside in similar deep water environmental conditions like Mamara Sea, Californian Tonya Seep, and others. Overall, the Thuregood group showed that lake stratification, and in this case extreme stratification, can illicit the formation of distinct microbial communities within the waterbody, An absence of vertical mixing can increase this effect, as the communities that inhabit the unmixable depths will grow and evolve in total isolation from the other communities. As the group moves forward they will turn their eye to the effects of the anthropogenic pollution and look to see if the overabundance of nutrients and heavy metals have changed the functional communities that inhabit the various niches of Landsort Deep. References 1. Thureborn P, Lundin D, Plathan J, Poole AM, Sjoberg BM, Sjoling S. 2013. A metagenomics transect into the deepest point of the baltic sea reveals clear stratification of microbial functional capacities. PloS one 8: e74983